Recently, high-speed wireless access systems using 2.4 GHz bands or 5 GHz bands, such as the standards of IEEE802.11g and IEEE802.11a, have been spreading remarkably. These systems may adopt orthogonal frequency division multiplexing (OFDM) modulation methods, i.e. technologies used to stabilize characteristics in multipath fading environments, so as to achieve a physical-layer transmission speed of maximally 54 Mbps (see Non-Patent Literature Document 1).
The above transmission speed indicates a transmission speed on a physical layer. In actuality, due to a transmission efficiency of 50-70% on a MAC (Medium Access Control) layer, the upper limit of an actual throughput may be limited to about 30 Mbps, and therefore characteristics may be further reduced due to the increasing number of wireless communication stations used to send information. The spread of wired LANs (Local Area Network) such as 100 Base-T interfaces of Ethernet (registered trademark) and FTTH (Fiber to Home) using optical fibers in households may promote the spread of high-speed lines at 100 Mbps, which in turn further increases transmission speeds in wireless LANs.
As technologies aiming for higher speeds, the IEEE802.11n standard introduced increased channel bandwidths and spatial multiplexing techniques (MIMO: Multiple Input Multiple Output). In drafting the IEEE802.11ac standard, it has been considered to employ further increased channel widths and multiuser MIMO (MU-MIMO) transmission methods adopting space division multiple access (SDMA) techniques expanding spatial multiplexing techniques (see Non-Patent Literature Document 2). In drafting the IEEE802.11ac standard, it has been considered to employ a new concept of group IDs (GID). Using group IDs, it is possible to concurrently transmit data to all of or part of wireless communication terminals belonging to a group specified via a GID field of each frame.
Among the technologies aiming for higher speeds, it is easy to facilitate speed increasing methods using increased channel bandwidths rather than spatial multiplexing techniques and space division multiple access techniques, and therefore speed increasing functions have been installed in numerous devices. For example, it is possible to increase speeds in such a way that the channel bandwidth of 20 MHz fixed to the IEEE802.11a standard is increased to 40 MHz in the IEEE802.11n standard. Additionally, the IEEE802.11 TGac (Task Group ac) has been currently working on a draft of the IEEE802.11ac standard considering an increase of the channel bandwidth to 80 MHz or 160 MHz. For example, it is possible to use two adjacent channels, each having the bandwidth of 20 MHz, for use in the total bandwidth of 40 MHz while it is possible to use four adjacent channels, each having the bandwidth of 20 MHz, for use in the total bandwidth of 80 MHz.
In wireless LAN systems based on the IEEE802.11 standards, wireless base station devices (which may be referred to as access points, hereinafter referred to as wireless base stations) having broadband transmission/reception abilities or functions at 40 MHz, 80 MHz, or 160 MHz may actually perform transmission/reception using channel bandwidths which should be limited to channel bandwidths supported by wireless communication terminal devices (hereinafter, referred to as wireless communication terminals) under command of wireless base stations. For wireless communication terminals unable to transmit or receive broadband signals at 40 MHz, 80 MHz, or 160 MHz, it is necessary for wireless base stations to transmit or receive data by use of available channel bandwidths for each wireless communication terminal.
The following description is made by taking an example of a wireless base station which is able to transmit or receive data via 80 MHz bands based on the standard (or draft) of IEEE802.11ac. At this time, it is possible for the wireless base station to transmit or receive data via 80 MHz bands with wireless communication terminals, employing 80 MHz modes based on the IEEE802.11ac standard (or draft), under command of the wireless base station. However, it is necessary for the wireless base station to transmit data on a single channel of 20 MHz with wireless communication terminals employing 20 MHz bands based on the IEEE802.11a standard.
In the systems based on IEEE802.11 standards, it is impossible to adequately demonstrate abilities of wireless base stations due to differences of channel bandwidths supported by wireless base stations and wireless communication terminals. Additionally, the increasing number of wireless communication terminals having low functions and abilities may degrade throughput characteristics and frequency usage efficiencies in the entirety of systems.
Next, a wireless data transmission/reception method adapted to a wireless LAN system based on IEEE802.11 will be described. In the wireless LAN system based on IEEE802.11 employing an access control procedure based on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance), wireless communication stations (hereinafter, wireless base stations and wireless communication terminals will be generally referred to as wireless communication stations) may avoid collision of signals with other wireless communication stations. A wireless communication station receiving a transmission request carries out a series of steps of: monitoring the states of wireless media during the predetermined sensing period (DIFS: Distributed Inter-Frame Space); assuming an unused state of channels (which may be referred to as an idle state) when other wireless communication stations do not transmit signals during the period; and then initiating a random backoff procedure (i.e. a process of deterring transmission for a waiting time which is used to control collision avoidance and determined based on a random number generated within the predetermined range). The wireless communication station continuously monitors wireless media during the random backoff period and then gets an exclusive channel transmission opportunity for the predetermined period (TXOP: Transmission Opportunity) when other wireless communication stations do not transmit signals during the random backoff period. The wireless communication station obtaining a transmission opportunity (TXOP) is called TXOP Holder (hereinafter, referred to as a TXOP-Holder wireless communication station). The wireless communication station serving as TXOP Holder may continuously transmit frames with very short time intervals, called SIFS (Short Inter-Frame Space), without performing CSMA/CA again in the TXOP period.
It is possible to provide “virtual carrier sense” as a method of solving concealed problems of terminals in wireless communications. Upon receiving frames including Duration (i.e. a continuous usable period) information used to notify a usable time of wireless media, wireless communication stations assume that wireless media are occupied in the period corresponding to Duration information (virtual carrier sense), and therefore wireless communication stations determine the period as a transmission suspension period (i.e. a NAV (Network Allocation Vector) period) so as to suspend transmitting frames in the NAV period. Thus, it is possible to guarantee exclusive usage of channels during the TXOP period.
A wireless communication station receiving a frame may implement NAV setting as necessary while simultaneously storing information (e.g. a MAC address) used to identify a TXOP-Holder wireless communication station, i.e. a wireless communication station serving as a source of transmitting the received frame indicating a frame used to start the TXOP period (see Non-Patent Literature Document 3). The wireless communication station deletes the information identifying a TXOP-Holder wireless communication station at the end of the TXOP period. As a frame used to start the TXOP period, it is unnecessary to use a special frame, but it is possible to use a signal which is used to reserve channels by transmitting a control frame such as an RTS (Request To Send) frame.
Upon receiving a frame again in the TXOP period, the wireless communication station confirms whether or not the address of the source of transmitting the received frame matches the MAC address which is stored as the information identifying a TXOP-Holder wireless communication station. When the source address matches the MAC address, the wireless communication station determines that a wireless communication station serving as a source of transmitting the received frame is identical to a TXOP-Holder wireless communication station, thus sending back a reply frame which is needed irrespective of the presence or absence of the NAV setting therein. This makes it possible for a TXOP-Holder wireless communication station to transmit or receive data with a plurality of wireless communication stations in the same TXOP period.
Hereinafter, operations of transmitting and receiving frames conducted between wireless communication stations will be described with reference to FIGS. 52 to 54. FIG. 52 is a schematic illustration of a wireless LAN cell A consisting of one wireless base station AP1 and three wireless communication terminals STA11 to STA13. Both the wireless base station AP1 and the wireless communication terminal STA13 based on the IEEE802.11ac standard are designed to support three types of transmission/reception bandwidths of 20 MHz, 40 MHz, and 80 MHz. The wireless communication terminal STA11 based on the IEEE802.11a standard is designed to support the transmission/reception bandwidth of 20 MHz while the wireless communication terminal STA12 based on the IEEE802.11n standard is designed to support the transmission/reception bandwidths of 20 MHz and 40 MHz.
FIG. 53 is a time chart showing the frame transmitting timings at which a TXOP-Holder wireless communication station transmits frames to other wireless communication stations in the TXOP period. In this drawing, the horizontal axis represents time. Symbols such as (STA11) described in each frame indicate wireless communication stations serving as destinations, for example, wherein (STA11) indicates the wireless communication terminal STA11 serving as a destination. Additionally, NAV(RTS) indicates the NAV setting after receiving RTS not destined to its own station. Among wireless communication stations corresponding to the wireless base station AP1 and the wireless communication terminals STA11 to STA13, the wireless base station AP1 collects data destined to the wireless communication terminals STA11 to STA13 so as to transmit frames to the wireless communication terminals STA11 to STA13. The wireless base station AP1 serving as TXOP Holder transmits data to the wireless communication terminal STA13, employing the largest band among terminals, on a channel of 80 MHz. Upon finishing data communication with the wireless communication terminal STA13, the wireless base station AP1 transmits data to the wireless communication terminal STA12 having the second largest band among terminals. Lastly, the wireless base station AP1 transmits data to the wireless communication terminal STA11 having the smallest band among terminals.
Hereinafter, the operations of the wireless base station AP1 and the wireless communication terminals STA11 to STA13 will be described with reference to FIG. 53. Upon generating data destined to the wireless communication terminals STA11 to STA13, the wireless base station AP1 carries out CSMA/CA so as to obtain a transmission opportunity (TXOP) by confirming and detecting no signals transmitted from other wireless communication stations. The wireless base station AP1 obtaining a transmission opportunity, serving as a TXOP-Holder wireless communication station (i.e. TXOP Holder), starts to transmit frames. The wireless base station AP1 transmits an RTS (Request TO Send) frame, serving as an initiate frame indicating the beginning of a frame sequence, to the wireless communication terminal STA13 utilizing the largest band among terminals sending data (time t111).
The wireless communication terminal STA13 sends back a CTS (Clear To Send) frame to the wireless base station AP1 since the received RTS frame is destined to its own station not having the setting of a transmission suspension period (time t112). Thus, the wireless communication terminal STA13 notifies the data-receivable state thereof to the wireless base station AP1.
The wireless communication terminals STA11 and STA12, i.e. other wireless communication stations receiving an RTS frame from the wireless base station AP1, set a NAV period (i.e. a transmission suspension period), indicated by continuous usage period information included in the RTS frame not destined to their own stations, so as to stop transmitting frames in the NAV period. Additionally the other wireless communication stations detect the beginning of the TXOP period (i.e. a usage transmission opportunity period) due to the RTS frame being received from the wireless base station AP1, while the wireless communication terminals STA11 to STA13 stores that the wireless base station AP1 serves as a TXOP-Holder wireless communication station (i.e. TXOP Holder).
Upon receiving a CTS frame from the wireless communication terminal STA13, the wireless base station AP1 transmits a frame destined to the wireless communication terminal STS13 (time t113). The wireless communication terminal STA13, properly receiving a frame destined to its own station, sends back a BA (Block ACK) frame (or an ACK (Acknowledgement) frame) (time t114), thus exiting transmission/reception of frames.
Next, the wireless base station AP1 transmits an RTS frame destined to the wireless communication terminal STA12 (time t115) in order to transmit data to the wireless communication terminal STA12 having the second largest band among terminals. Herein, the wireless communication terminal STA12, already setting NAV in its own station, receives a frame from TXOP Holder so as to send back a CTS frame destined to a wireless base station AP1 serving as TXOP Holder (time t116).
The wireless communication terminals STA11 and STA13 receive an RTS frame destined to other wireless communication terminals so as to set a NAV period. When the NAV period is set in advance, they update the NAV value. Upon properly receiving a CTS frame from the wireless communication terminal STA12, the wireless base station AP1 transmits a frame to the wireless communication terminal STA12 (time t117). Upon properly receiving a frame from the wireless base station AP1, the wireless communication terminal STA12 sends back a BA frame (or an ACK frame) to the wireless base station AP1 (time t118), thus exiting transmission/reception of frames.
Next, the wireless base station AP1 transmits an RTS frame destined to the wireless communication terminal STA11 (time t119) in order to transmit data to the wireless communication terminal STA11 having the smallest band among terminals. The wireless communication terminal STA11 receives an RTS frame from the wireless base station AP1, serving as a TXOP-Holder wireless communication station, so as to send back a CTS frame to the TXOP-Holder wireless communication station (time t120) regardless of whether or not the current period belongs to the NAV period.
The wireless communication terminals STA12 and STA13 receive an RTS frame not destined to their own stations so as to set a NAV period. When the NAV period is set in advance, they update the NAV value. Upon properly receiving a CTS frame from the wireless communication terminal STA11, the wireless base station AP1 transmits a frame to the wireless communication terminal STA11 (time t121). Upon properly receiving a frame from the wireless base station AP1, the wireless communication terminal STP11 sends back a BA frame (or an ACK frame) to the wireless base station AP1 (time t122), thus exiting transmission/reception of frames.
The above description exemplifies a frame sequence employing a MAC protection method using RTS/CTS exchange before data transmission, but it is possible to transmit frames just after obtaining an access right without RTS/CTS exchange. Additionally, the above description shows an example of transmitting data to a plurality of terminals in the same TXOP period. As described above, it is possible to transmit frames to a plurality of terminals within a range of periods not exceeding the upper limit of TXOP defined based on the IEEE802.11 standards. In this case, it is impossible to perform communication using channel bandwidths larger than channel bandwidths once used in the TXOP period. That is, it is impossible to broaden channel bandwidths used in the TXOP period, but it is possible to reduce channel bandwidths as necessary. In the case of FIG. 53, it is necessary to transmit frames in an order of destinations having larger channel bandwidths among the wireless communication terminal STA11 having Channel 1 (CH1), the wireless communication terminal STA12 having CH1 and CH2, and the wireless communication terminal STA13 having CH1 to CH4.
Next, channel bandwidths used for data transmission among the wireless base station AP1 and the wireless communication terminals STA11 to STA13 will be described with reference to FIG. 54. FIG. 54 shows channel bandwidths used for data transmission among the wireless base station AP1 and the wireless communication terminals STA11 to STA13. The wireless base station AP1 communicates with the wireless communication terminal STA11, solely using 20 MHz bands, on Channel 1 (CH1).
Non-Patent Literature Document 3 defines unit channels, called primary channels, which should be necessarily used to perform communication in a cell configured of a certain access point and terminal stations. Other channels, which are not primary channels but used for communication, are called secondary channels. Alternatively, Non-Patent Literature Document 2 names other channels as secondary xMHz channels (where x indicates any one of 20, 40, 80). In the present specification, arbitrarily unit channels, which are not primary channels within the entire bandwidth of each cell, will be referred to as secondary channels. FIG. 55 shows examples of primary channels and secondary channels on the condition that each unit channel has a 20 MHz band within each cell having the entire bandwidth of 80 MHz. FIG. 55 shows three secondary channels.
The wireless base station AP1 may communicate with the wireless communication terminal STA12, adaptable up to 40 MHz bands, on a primary channel of 20 MHz and another 20 MHz channel (i.e. a secondary channel) adjacent to the primary channel (i.e. CH1 and CH2). Additionally, the wireless base station AP1 may communicate with the wireless communication terminal STA13, adaptable up to 80 MHz bands, on a primary channel and three secondary channels.